This invention pertains generally to mass spectrometry analysis techniques and more particularly to secondary ion mass spectrometry (SIMS) analysis techniques.
Gas chromatography coupled with mass spectrometry (GC/MS) is often the method used to analyze mixtures of organic compounds. The compounds are separated after volatilizing the molecules in the gas chromatrographic process and identified using mass spectrometry. The separation is necessary to purify the components and, thus, reduce interference in the mass spectrometric technique. This technique works well for small, volatile molecules that can easily withstand the temperatures necessary to vaporize the molecule without being thermally degraded. However, large, involatile, thermally fragile molecules, particularly enzymes, proteins, drugs, and other similar molecules of biological interest, can not be analyzed using this technique because they would be degraded by the heat necessary to volatilize the molecules for the chromatography process.
Other methods have also been used to produce molecules which can be analyzed using MS. Electron ionization (EI) produces a positive ion radical by stripping away an electron from the parent molecule The molecular ion and its fragments give a mass spectrum characteristic of the molecule. Similarly, Munson and Field, Chemical Ionization Mass Spectrometry, 88 J. Am. Chem. Soc. 2621 (1966), analyzed molecules using a chemical ionization (CI) technique which used ions, particularly protons, to generate molecular species which could be analyzed using mass spectrometry CI and EI, however, generally require that the ionizing agent act on gas-phase molecules. Thermally fragile molecules required the development of new methods which would allow mass spectrometric analysis without degrading the molecule.
New techniques permitting analysis of large, involatile, and thermally fragile molecules have recently been developed. Mass spectrometric analysis was accomplished by directly desorbing the molecular ions from the solid or liquid phase. This eliminated the need to vaporize the molecules or ions thus reducing thermal degradation. This general technique, termed desorption ionization (DI), has lead to the discovery of several additional desorption methods These include secondary ion mass spectrometry (SIMS), fast-atom bombardment (FAB), plasma desorption (PD), field desorption (FD), electrohydrodynamic ionization (EMHS) and thermal desorption. These techniques are reviewed and further defined in Busch and Cooks, Mass Spectrometry of Large, Fragile, and Involatile Molecules, 218 Science 247 (1982).
SIMS, the subject of the present invention, desorbs and ionizes molecules in the solid form or mixed with a solid matrix using energetic ions. Low fluxes of ions are used for surface analysis of organic molecules thus avoiding ion degradation of the molecules. Higher fluxes are used for inorganic compounds or for depth profiling. SIMS, therefore, provides a highly sensitive method of analysis for large, volatile, thermally fragile molecules. The experiment is simple compared to GC/MS and other techniques. The samples analyzed can often be observed for hours under low flux bombardment.
Desorption of ions from the solid matrix requires that ions be generated, usually by ion impact, fission particle impact, laser irradiation, chemical manipulation, and similar techniques. These methods, however, are limited by their lack of specificity for particular molecules that may be in a mixture. Ion impact, fission particle impact, laser irradiation, and similar techniques ionize the molecule they contact, whether it be the molecule of interest or another molecule. Similarly, chemical manipulation, principally accomplished by the addition or removal of a proton, is non-specific and affects any molecule with a protonated functional group. Novel methods of selectively ionizing a molecule of particular interest could, therefore, greatly increase the efficacy of the SIMS technique.